A piezoelectric actuator employing a piezoelectric stack is sometimes used as a driving source of a fuel injection device for an internal combustion engine of a vehicle such as an automobile.
The piezoelectric stack within the piezoelectric actuator is hermetically sealed in an airtight package to protect it from high-temperature fuel or moisture.
However, the piezoelectric layer of the piezoelectric stack comprises a polycrystalline ceramic and, accordingly, a large number of crystal grain boundaries are present inside the piezoelectric layer. The crystal grain boundary includes gaps as shown in FIG. 6 which is described later.
When the piezoelectric stack is exposed to a high-temperature and closed environment, originally adsorbed moisture, a decomposition product of the constituent resin material, and various substances constituting the internal electrode layer and piezoelectric layer sometimes intrude into the crystal grain boundary and by this phenomenon, the physical properties such as insulation resistance value of the piezoelectric layer are changed.
This phenomenon gives rise to deterioration in the performance or reduction in the reliability of the piezoelectric stack.
Particularly, when the intruded substance is an electrically conducting substance, this may incur a decrease in the insulation resistance value of the piezoelectric layer.
In JP-2001181041-A as a prior art, a glass phase is present only at the triple point of grains and is not present at most of other crystal grain boundaries. Therefore, an electrically conducting substance may intrude into a crystal grain boundary having no glass phase to decrease the insulation resistance value.
An object of the present invention is to provide a piezoelectric stack substantially free from deterioration in the performance and having high reliability, and a production method thereof.